Fibre optic based semiconductor micro sensors for sensing pressure or temperature, fabrication methods of said sensors, and a method of securing an optical fibre to a silicon block

ABSTRACT

An optical micro sensor ( 1 ) for measuring one or more environmental parameters, such as pressure and temperature, through the modification of incident radiation. The sensor ( 1 ) is fabricated using MEMS technology and is adapted to receive an optical fibre ( 40 ) which communicates radiation to and from the micro sensor ( 1 ). The sensor ( 1 ) has an environmentally-sensitive element ( 4 ) which modifies the incident radiation communicated by the optical fibre ( 40 ). The modified radiation is communicated back along the optical fibre ( 40 ) and provides information regarding the environmental conditions surrounding the sensor ( 1 ). The pressure sensor is provided with a Fabry Perot cavity ( 3 ) in a first surface of a silicon wafer ( 2 ). The cavity is covered by a reflector at the environmentally-sensitive element ( 4 ). The diameter of the channel ( 7 ) holding the optical fibre ( 40 ) is greater than the diameter of the cavity. ( 3 ). The temperature sensor is provided with luminescent material at the element ( 4 ). Also, a method of securing an optical fibre to a silicon block is claimed.

The present invention relates to an optical sensor for measuring one ormore parameters through the modification of incident radiation and tothe fabrication of such a sensor. The present invention particularly butnot exclusively relates to micro devices such as amicroelectromechanical system (MEMS) device adapted to receive anoptical fibre and to modify radiation communicated via the opticalfibre, the modification being dependent upon the environmentalconditions including but not limited to pressure, temperature, fluidflow, pH, oxygen concentration, carbon dioxide concentration, glucoseconcentration, lactate concentration, bicarbonate ion concentration,chlorine ion concentration, sodium and potassium ion concentration, towhich the MEMS is exposed.

In “Micromachined fibre optic Fabry-Perot pressure sensors foraerodynamics applications”, M J Gander et al., Applied Optics andOptoelectronics Conference, 2-5 September 2002, Cardiff, U.K., a sensorutilising a Fabry-Perot microcavity was described for measuringpressure. The Fabry-Perot cavity is formed by etching a well in thesurface of a silicon substrate; the depth of the well corresponding tothe length of the Fabry-Perot cavity. A deep second etch is thenperformed on an area of the silicon substrate encompassing the well. Thewell profile is approximately maintained during the deep etch whichcontinues until the bottom surface of the well is completely removed anda copper film, disposed on the opposing surface of the siliconsubstrate, is revealed. This deep etch creates a channel having adiameter sufficient to accommodate an optical fibre. When the opticalfibre is inserted into the channel, the well profile describes aFabry-Perot cavity with the facing surfaces of the optical fibre and thecopper film forming the reflective surfaces. Unfortunately, during thedeep second etch the profile of the well structure is not accuratelymaintained. In particular, the depth of the well which determines thecavity length, cannot accurately nor predictably be formed. Accordingly,each sensor manufactured in this way requires calibration. Similarly,the diameter of the well cannot accurately nor predictably be formed.Accordingly, the exposed surface area of the copper film, which acts asthe pressure membrane, and thus the response of the pressure sensor mayalso vary.

An alternative optical sensor for measuring temperature as well aspressure is described in EP-A-0392897. The sensor comprises ahemispherical elastomeric material which is attached to the end of theoptical fibre. A reflective coating is provided on a region of theconvex surface of the elastomeric material and a luminescent material isapplied over the reflective coating and the remaining regions of theconvex surface of the elastomeric material. On application of externalpressure, the hemispherical elastomeric material is deformed and thusthe degree of optical coupling between the reflective surface and theend of the optical fibre is changed. Accordingly, the intensity of thelight reflected from the reflective coating changes. However, theelastic properties of the elastomeric material vary with temperature andthus the sensor is unsuitable for measuring pressure variations over alarge temperature range.

Furthermore, excitation radiation striking the luminescent materialcauses the material to luminesce. The characteristics of the luminescentemissions communicated back along the optical fibre are temperaturedependent and so monitoring the luminescent emissions provides a measureof the sensor's temperature. The sensor is of course dependent upon theintensity characteristics of the luminescent emissions transmitted bythe optical fibre. However, the optical coupling between the opticalfibre and the luminescent material is poor thus reducing the amount oflight captured by the optical fibre. This problem is particularly acutewhere the sensor is required to respond to relatively rapid changes intemperature as in these circumstances the layer of luminescent materialmust be kept relatively thin e.g. a few microns, which further reducesthe intensity of the luminescent emissions.

It is, of course, possible to achieve a good optical coupling where theend surface of the optical fibre is directly coated with luminescentmaterial. However, this arrangement also suffers from low emissionintensity because only emissions from the luminescent material incontact with the small fibre core is collected.

Moreover, commonly an optical fibre is secured to a micro sensor such asa MEMS sensor by means of an adhesive applied to the cladding of theoptical fibre and the surface of the silicon substrate. However,repeated small displacements of the optical fibre with respect to thesubstrate over time can cause the adhesive to become separated from thesubstrate surface.

It is an object of this invention to provide a micro optical sensorwhich overcomes in part one or more of the aforementioned disadvantagesof the prior art.

It is a further object to provide a micro optical sensor suitable formeasuring a multiplicity of parameters including but not limited topressure, temperature, fluid flow, pH, oxygen concentration, carbondioxide concentration, glucose concentration, lactate concentration,bicarbonate ion concentration, chlorine ion concentration, sodium andpotassium ion concentration.

It is further object to provide a low-cost, disposable micro opticalsensor preferably having quick (sub-minute) reaction times and beingsuitable for in vivo medical applications.

Accordingly, in a first aspect the present invention provides a methodfabricating an optical sensor comprising the steps of: providing asilicon substrate having a first surface and a second surface; providinga region comprising essentially of silicon dioxide on or in the firstsurface of the silicon substrate; etching a channel into the siliconsubstrate from said second surface up to said silicon dioxide region,said channel being sized to receive an optical fibre whereby saidsilicon dioxide region forms an end portion of said channel which atleast partially closes said channel; and coating at least a portion ofthe silicon dioxide region with a coating to form anenvironmentally-sensitive element.

Preferably, the silicon substrate and silicon dioxide region form asingle substrate element requiring no additional substrate elements toform the optical sensor.

The silicon substrate is preferably monolithic.

The step of providing the silicon dioxide region preferably compriseseither oxidising a portion of the first surface of the siliconsubstrate, or etching at least one continuous groove, preferably anannular groove, in the first surface of the silicon substrate andthereafter forming silicon dioxide in the at least one groove.

The optical coupling may be provided by etching an aperture in thesilicon dioxide region or, alternatively, by forming at least oneprojection comprising essentially of silicon dioxide on said silicondioxide region. The projection may be formed by providing a layer ofsilicon over the silicon dioxide region; etching the layer of thesilicon to form at least one structure projecting outwardly from saidsilicon dioxide region; and thereafter oxidising the at least onestructure to form said at least one projection. Etching the layer ofsilicon may comprise etching at least two concentric grooves to form oneor more annular projecting walls or, alternatively, etching two or morelinear parallel grooves to form at least one planar projecting wall.

The step of providing an environmentally-sensitive element may compriseattaching a pressure-sensitive membrane having a radiation-reflectivesurface to the silicon dioxide region and etching a through-hole in thesilicon dioxide region from the channel to the pressure-sensitivemembrane. Alternatively, the environmentally-sensitive element may beprovided by coating at least a portion of the at least one projectionwith luminescent material.

In a second aspect, the present invention provides a method ofmanufacturing a sensor comprising the steps of: providing a siliconwafer; forming at least one continuous groove in a first surface of thesilicon wafer; applying or forming silicon dioxide in said continuousgroove and over the surface area of the silicon wafer encompassed by thecontinuous groove to form a layer of silicon dioxide; applying areflector over at least the layer of silicon dioxide encompassed by thecontinuous groove; forming a channel in the silicon wafer extending froman opposed second surface of the silicon wafer up to the layer ofsilicon dioxide and forming a cavity beyond the channel in which thewall of the cavity is defined by the silicon dioxide provided in thecontinuous groove, the longitudinal axis of the channel and the cavitysubstantially intersecting the centre of the path followed by thecontinuous groove; and removing at least the silicon dioxide layerencompassed by the continuous groove thereby exposing a reflectivesurface of said reflector.

The continuous groove in the silicon wafer may be in the form of asubstantially circular groove provided by applying a mask having a ringaperture over the first surface of the silicon wafer and etching exposedregions of the first surface.

Preferably, a plurality of concentric continuous grooves are formed inthe first surface of the silicon wafer, each concentric groove having athickness and being separation from an adjacent groove a distanceselected such that oxidation of the first surface of the silicon waferincluding the plurality of concentric grooves produces a silicon dioxidetorus in the first surface of the silicon wafer.

The reflector is preferably applied by coating at least the silicondioxide layer encompassed by the continuous groove with a thin metallicfilm or thin film stack.

In a third aspect, the present invention provides a method ofmanufacturing a sensor comprising the steps of: providing asilicon-silicon dioxide-silicon wafer; etching an upper surface of thesilicon-silicon dioxide-silicon wafer to form at least one siliconstructure projecting from the silicon dioxide layer of the wafer,oxidising at least a portion of the exposed upper surface of the waferincluding the silicon structure so as to form at least one silicondioxide projection; coating at least a portion of the silicon dioxideprojection with a luminescent material; and forming a channel in theopposed surface of the silicon-silicon dioxide-silicon wafer as far asthe silicon dioxide layer, the longitudinal axis of the channel beingsubstantially aligned with the silicon dioxide projection.

In a fourth aspect, the present invention provides a method offabricating a plurality of optical sensors on a common substratecomprising the steps of: providing a silicon substrate having a firstsurface and a second surface; providing at least two regions eachcomprising essentially of silicon dioxide on or in the first surface ofthe silicon substrate; providing optical couplings each associated witha respective silicon dioxide region; providing at least oneenvironmentally-sensitive element for optical coupling with a respectiveoptical fibre by means of a respective one of the optical couplings; andetching at least one channel into the silicon substrate from said secondsurface up to one or more of said silicon dioxide regions, the channelbeing sized to receive an optical fibre whereby said one or more silicondioxide region forms an end portion of the channel which at leastpartially closes said channel.

The step of etching at least one channel may comprise etching aplurality of channels, each channel being sized to receive an opticalfibre and whereby each silicon dioxide region forms an end portion ofthe respective channel which at least partially closes said channel.Alternatively, the step of etching at least one channel may compriseetching a single channel into the silicon substrate from said secondsurface up to all said silicon dioxide regions, the channel being sizedto receive an optical fibre whereby the silicon dioxide regions form anend portion of the channel which at least partially closes said channel.

The silicon substrate is preferably monolithic.

In a fifth aspect, the present invention provides an optical sensorcomprising: a silicon substrate having a first surface and an opposedsecond surface; a channel extending into the silicon substrate from saidsecond surface, said channel being sized to receive an optical fibre andhaving an end portion distant from said second surface, said end portionat least partially closing said channel and comprising essentially ofsilicon dioxide; and an optical coupling associated with said endportion of said channel.

The optical sensor preferably comprises an environmentally-sensitiveelement associated with the optical coupling.

The optical coupling of the sensor may comprise a through-hole in theend portion of the channel. In one particular embodiment, theenvironmentally-sensitive element is preferably a pressure-sensitivemembrane provided at the first surface of the silicon substrate with thesurface of the pressure-sensitive membrane facing towards the channelbeing reflective to incident radiation.

In an alternative embodiment, the optical coupling may comprise at leastone projection consisting essentially of silicon dioxide provided on theend portion of the channel. In this embodiment, theenvironmentally-sensitive element is preferably a luminescent coatingapplied over at least a portion of the at least one projection.

In a sixth aspect, the present invention provides a sensor comprising asilicon wafer having a cavity in a first surface covered by a reflectorand a channel extending from an opposed second surface of the siliconwafer to the cavity and being in communication therewith, the diameterof the channel being greater than the diameter of the cavity and the endof the channel adjacent the cavity comprising essentially of silicondioxide.

The reflector is preferably a thin metallic film or a thin-filmmulti-layered dielectric stack.

In a seventh aspect, the present invention provides a sensor comprisinga silicon wafer having at least a region of a first surface of thesilicon wafer covered by a layer of silicon dioxide and at least onestructure comprising essentially of silicon dioxide projecting outwardlyfrom the silicon dioxide layer and having a luminescent materialcovering at least a portion of said silicon dioxide structure and achannel extending from an opposed second surface of the silicon wafer tosaid silicon dioxide layer and aligned with said silicon dioxidestructure.

In an eighth aspect, the present invention provides a sensor systemcomprising a plurality of optical sensors on a common substrate having afirst surface and an opposing second surface, each optical sensorcomprising: a channel extending into the common substrate from saidsecond surface, said channel being sized to receive an optical fibre andhaving an end portion distant from said second surface, said end portionat least partially closing said channel and comprising essentially ofsilicon dioxide; and an optical coupling associated with said endportion of said channel; at least one of said optical sensors furthercomprising an environmentally-sensitive element for optical couplingwith an optical fibre by means of said optical coupling.

In a ninth aspect, the present invention provides a sensor systemcomprising a plurality of optical sensors on a common substrate having afirst surface and an opposing second surface and a channel extendinginto the common substrate from said second surface, said channel beingsized to receive an optical fibre, each optical sensor comprising: anend portion distant from said second surface at least partially closingsaid channel and comprising essentially of silicon dioxide; and anoptical coupling associated with said end portion; at least one of saidoptical sensors further comprising an environmentally-sensitive elementfor optical coupling with an optical fibre by means of said opticalcoupling.

In a tenth aspect, the present invention provides a sensor systemcomprising a common substrate of monolithic silicon having a firstsurface and an opposing second surface and at least one pressure sensorand at least one optical sensor for measuring a parameter selected from,but not limited to, temperature, fluid flow, pH, oxygen concentration,carbon dioxide concentration, glucose concentration, lactateconcentration, bicarbonate ion concentration, chlorine ionconcentration, sodium and potassium ion concentration, the pressuresensor comprising: a cavity formed in the first surface of the substratecovered by a reflector and a channel extending from the second surfaceof the substrate to the cavity and being in communication therewith, thediameter of the channel being greater than the diameter of the cavityand the end of the channel adjacent the cavity comprising essentially ofsilicon dioxide; and the optical sensor comprising: a layer of silicondioxide covering at least a region of the first surface of the substrateand at least one structure comprising essentially of silicon dioxideprojecting outwardly from the silicon dioxide layer and having aluminescent material covering at least a portion of said silicon dioxidestructure and a channel extending from the second surface of thesubstrate to said silicon dioxide layer and aligned with said silicondioxide structure.

In an eleventh aspect, the present invention provides a method ofsecuring an optical fibre to a silicon block, the method comprising thesteps of: forming a channel extending into the silicon block from asurface of the block, the channel being sized so as to accommodate anend of the optical fibre; forming an aperture in the surface of thesilicon block adjacent the opening of the channel in the surface of thesilicon block; inserting an optical fibre into the channel; and applyingan adhesive to the optical fibre and to the surface of the silicon blockadjacent the optical fibre and including into the adjacent aperture.

The aperture is preferably an annular groove encircling the opening tothe channel. Furthermore, the width of the aperture at the surface ofthe silicon block is preferably less than the width of the aperturebelow the surface of the silicon block

The sensors of the present invention are particularly well adapted foruse in medicine applications where sensors having quick response times(sub-minute) can prove critical. With this invention sensors formonitoring blood pressure, temperature, fluid flow, pH, oxygenconcentration, carbon dioxide concentration, glucose concentration,lactate concentration, bicarbonate ion concentration, chlorine ionconcentration, sodium and potassium ion concentration, for example, canbe provided on the end of a single probe. With the present inventionsensor arrays having a sub-millimetre diameter can be fabricated whichmakes these sensors suitable for mounting on the end of in vivo probessuch as endoscopes.

In particular, the sensors may be used in in vivo patient monitoring,such as that performed in intensive care units, elective surgery andanaesthetics. The sensors may also be used in conjunction with drugdelivery systems arranged in feedback, such that drug delivery iscontrolled according to measurements made by the sensors. The sensorsmay also be used to assess particular medical conditions. For example,metabolic and hormone levels can be monitored using these sensors whichis of use in assessing fertility levels.

The sensors may also be of use in the field of psychiatry by measuring,for example, the levels of neurotransmitters such as serotonin

The sensors of the present invention are also well adapted for use infood processing, for example, the sensors may be used to probefoodstuffs to measure temperature and/or pH levels.

As the sensors are preferably constructed from monolithic silicon, theycan be manufactured using conventional techniques employed in MEMSmanufacture. This makes the sensors suitable for mass production.Accordingly, disposable, sterile sensors having low production costs maybe manufactured.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a schematic cross sectional view of a micro pressure sensor inaccordance with a first embodiment of the present invention;

FIG. 2 illustrates a first step in the process of manufacturing thesensor of FIG. 1;

FIG. 3 illustrates a further step in the process of manufacturing thesensor of FIG. 1;

FIG. 4 illustrates a further step in the process of manufacturing thesensor of FIG. 1;

FIG. 5 illustrates a further step in the process of manufacturing thesensor of FIG. 1;

FIG. 6 is a schematic cross sectional view of a micro pressure sensor inaccordance with a second embodiment the present invention;

FIG. 7 illustrates a first step in the process of manufacturing thesensor of FIG. 6;

FIG. 8 illustrates a further step in the process of manufacturing thesensor of FIG. 6;

FIG. 9 illustrates a further step in the process of manufacturing thesensor of FIG. 6;

FIG. 10 is a schematic cross sectional view of a micro pressure sensorin accordance with a third embodiment the present invention;

FIG. 11 illustrates a first step in the process of manufacturing thesensor of FIG. 10;

FIG. 12 illustrates a further step in the process of manufacturing thesensor of FIG. 10;

FIG. 13 illustrates a further step in the process of manufacturing thesensor of FIG. 10;

FIG. 14 illustrates an alternative step in the process of manufacturingthe sensor of FIG. 10;

FIG. 15 is a schematic cross sectional view of a micro temperaturesensor in accordance with a first embodiment of the present invention;

FIG. 16 illustrates a step in the process of manufacturing the sensor ofFIG. 15;

FIG. 17 illustrates a further step in the process of manufacturing asensor of FIG. 15;

FIG. 18 is a schematic cross sectional view of a micro temperaturesensor in accordance with a second embodiment of the present invention;

FIG. 19 illustrates a step in the process of manufacturing the sensor ofFIG. 18;

FIG. 20 illustrates a further step in the process of manufacturing asensor of FIG. 18; and

FIG. 21 is a schematic cross sectional view of an annular groove formedon the surface of a substrate in accordance with present invention.

The micro sensor 1 illustrated in FIG. 1 is adapted to measure fluidpressure (gas or liquid, static or flowing) to which the sensor 1 isexposed. The sensor 1 is constructed from a silicon wafer and consistsof a sensor body 2 having a Fabry-Perot cavity 3 and a membrane 4disposed at a first end 5 of the cavity. The opposed second end 6 of thecavity 3 is in communication with a channel 7 adapted to receive anoptical fibre. The sensor body 2 includes a shoulder 8 which encirclesthe second end 6 of the cavity 3 and which abuts and supports the end ofthe optical fibre inserted into the channel 7. In this example thesensor 1 has a cavity length of 30 μm and the channel 7 is sized toreceive an optical fibre having an external diameter of 125 μm and acore diameter of 10 μm. It will be appreciated, however, that the sensormay be constructed with a different cavity length and be adapted toreceiving other sizes of optical fibre.

As shown in FIG. 2, the first step of fabrication involves forming thesupporting shoulder 8 of the sensor. With this particular fabricationmethod a series of concentric continuous grooves 13, for example annulargrooves, are formed in a first (upper) surface 12 of a silicon substrateor wafer 11. Five concentric grooves 13 are illustrated having acollective inner and outer diameter of approximately 60 μm and 150 μmrespectively. The profile of each groove 13 is approximately rectangularin cross-section with a depth of 30 μm and a radial width ofapproximately 5 μm Each groove 13 is separated from each neighbouringgroove 13 by a radial distance of approximately 5 μm. Adjacent grooves13 define a concentric ridge 16 with each concentric ridge 16therebetween similarly having a generally rectangular profile with aheight of 30 μm and a radial width of 5 μm.

The grooves 13 are formed by applying a photoresist mask 10 patternedwith a series of concentric rings over the surface 12 of the siliconsubstrate 11 and etching the exposed surfaces using deep reactive ionetching (DRIE). Other masks, both soft and hard, may alternatively beused, e.g. silicon dioxide or silicon nitride. Whilst DRIE is thefavoured method of etching due to its relatively high etch rates,alternative forms of etching, both wet and dry, may nevertheless beemployed, e.g. photoenhanced wet chemical etching, KOH, sputter etching,vapour phase etching. Furthermore, it is not necessary that the etchingprocess is anisotropic. Indeed, the cross-sectional profile of eachgroove 13 is preferably open such that an obtuse angle is formed by thesidewall 14 of each groove 13 and the base 15 of the groove 13. Inhaving a groove with an open profile, the risk of starving the base 15of the groove 13 of oxygen during the oxidation process (see below) issignificantly reduced. A groove 13 having an open profile is achievedusing DRIE by varying process parameters such as etchant and passivationgases, process pressures, switching ratios, coil and platen powers for aDRIE-type process. Alternatively, open profile grooves may be achievedby anisotropic wet etching of a single crystal of silicon, e g.photo-assisted and/or electrical-assisted wet chemical etching. Theprofile of each groove 13 is preferably such that the radial width atthe base of each ridge 16 is 6 μm whilst the radial width at the top ofeach ridge 16 is 4 μm.

The photoresist mask 10 is then removed from the surface 12 of thesilicon substrate 11 and the surface 12 is oxidised such that a layer ofsilicon dioxide 19 is formed over the upper surface of the siliconsubstrate and in particular over at least the grooved region of thesilicon substrate and the region of the silicon substrate surface 17located within, i.e. encompassed by the innermost concentric groove. Theoxidation process is controlled such that only a 2 μm layer normal tothe surface 12 of the silicon substrate 11 is oxidised. A silicon layerof thickness d will generally form a silicon dioxide layer of thickness2.27 d upon oxidation. Accordingly, a silicon dioxide layer ofapproximately 4.55 μm in thickness is formed over at least the groovedregions and the region of the substrate 17 enclosed by the innermostconcentric groove.

As is demonstrated in FIG. 3, the oxidation process results in a siliconsubstrate 11 having a single ring or torus 18 of silicon dioxide in itsupper surface 12 which functions as a fibre supporting structure. Thering 18 generally has an inner diameter of 56 μm, an outer diameter of154 μm and a depth of 34.55 μm. Also, over the top of the siliconsubstrate encompassed by the ring 18 lies a silicon dioxide layer 19 ofaround 4.5 μm in thickness. As the ring 18 in the surface 12 of thesilicon substrate 11 was formed from the oxidation of the fourconcentric ridges 16 the silicon-silicon dioxide interface formed duringoxidation may not be entirely planar along the base of the ring 18.Instead, the silicon-silicon dioxide interface may be rippled, asillustrated, with peaks of silicon immediately beneath where each of theformer ridges was located. Similarly, the upper surface of the ring mayhave a corresponding ripple, as illustrated. In the Figures, therippling is exaggerated for ease of identification.

Wet thermal oxidation is the preferred method of oxidation as itencourages fusion of the concentric ridges 16. In addition, the rates ofoxidation that can be achieved are relatively fast. Whilst other formsof oxidation, such as wet anodisation, chemical vapour deposition orplasma oxidation, may alternatively be employed, the concentric ridges16 do not fuse and the ring 18 is thus less strong. It is important thatthe oxidised ring 18 has a high etch selectivity to silicon when exposedto DRIE such that it provides an etch stop. Nevertheless, alternativeforms of oxidation may be employed in addition to wet thermal oxidationto fill any exposed voids in the oxide ring 18.

Turning now to FIG. 4, after oxidation, a reflector layer in the form ofa metallic thin film 30 is deposited over at least the surface of thesilicon dioxide layer 19 enclosed by the silicon dioxide ring 18. Themetallic thin film is preferably aluminium or copper and is preferablyof a few hundred Angstroms thick. The film may be deposited usingconventional deposition techniques, e.g. sputtering or chemical vapourdeposition. The choice of metal and thickness of the film will dependupon the desired pressure range and sensitivity of the sensor. Thechoice of metal may also be dictated to some extent by the wavelengthcharacteristics of the radiation that will be reflected from the surfaceof the film. Whilst reference herein has been to a metallic thin filmfor reflecting the incident radiation, alternative materials such asmultilayer dielectric stacks or dichroic dielectric materials maysimilarly be used as the reflecting material. Again, conventionalmethods may be employed for applying these materials over the silicondioxide layer 19.

Following application of the reflector layer 30, a channel 22 is etchedinto the opposed second surface 12′ of the silicon substrate 11. Thechannel 22 is generally cylindrical in shape and has a diametersufficient to receive an optical fibre. The channel 22 extends into thesilicon substrate 11 as far as the layer 19 of silicon dioxide. Thus,the silicon dioxide layer is used as the etch stop and produces achannel the end of which comprises essentially of the silicon dioxide.The end of the channel is only partially closed so that it maycommunicate with a cavity 3 defined by the internal diameter of thesilicon dioxide ring 18 and the section 24 of the silicon dioxide layerencompassed by the ring 18. The longitudinal axis of the channel 22 issubstantially normal to the innermost surface 23 of the silicon dioxidedisc 24 enclosed by the innermost wall 25 of the ring 18. Furthermore,preferably the intersection of the longitudinal axis of the channel 22with the silicon dioxide surface 23 is substantially coincident with thecentre of the innermost wall 25 of the ring 18. As a result of thediameter of the channel 22, which is preferably 126-130 μm in order toaccommodate the optical fibre, and the diameter of the cavity 3, aportion of the ring 18 is exposed as a result of the etch and acts as anannular shoulder at the inner end of the channel 22 for abutment with anend of the optical fibre when inserted 25 into the channel 22. In thisway the end of the optical fibre forms the final, closing, wall of thecavity 3.

The channel 22 is preferably formed using DRIE owing to the relativelyhigh etch rates and the ability to achieve almost vertical sidewalls.Again, alternative forms of anisotropic etching may be employed.

To aid assembly of the sensor, the opening of the channel at the secondsurface 12′ of the silicon substrate is tapered so as to assist fibrealignment. Tapering of the channel is achieved by first carving out anisotropic reactive ion etch on the opposed second surface 12′ of thesilicon substrate 11 followed by an anisotropic deep reactive ion etch.

Finally, as illustrated in FIG. 5, the exposed silicon dioxide at theinner end of the channel 22 and in the cavity 3 is etched usingconventional etching processes until the surface of the reflector 30 isexposed. Preferably, the entire silicon dioxide disc 24 enclosed by theinnermost wall 25 of the ring 18 is removed. However, in some instancesit may be desirable to remove only a portion of the silicon dioxide disc24.

The channel 22 thus extends from an open second end 12′ of the siliconsubstrate 11, through the substrate to a cavity and to a closed cavityend formed by the innermost surface of the reflector 30. Within thechannel 22 is an annular shoulder formed by an exposed portion of thesilicon dioxide ring 18. The distance between the fibre abutting surfaceof the annular shoulder and the innermost surface of the reflector 30,along the longitudinal axis of the channel 22 is the cavity lengthwhich, with this fabrication method, is accurately fabricated to 30 μmor whatever cavity length is required.

In the final construction of the pressure sensor, with reference to FIG.5, an optical fibre 40 is inserted into the sensor 10 via the open endof the channel 22 and pushed along the channel 22 until the end of thefibre 41 abuts the annular shoulder 18. At this point, the end 41 of theoptical fibre 40, the innermost annular wall 25 of the ring 18 and theinnermost surface 31 of the reflector 30 form a Fabry-Perot cavity. Thecavity length is defined by the distance between the end surface 41 ofthe optical fibre 40 and the innermost surface 31 of the reflector 30,parallel to the longitudinal axis of the optical fibre.

In the above-described example, the initial concentric grooves 13 formedin the silicon substrate 11 were chosen to have a collective inner andouter diameter of 60 μm and 150 μm respectively. The diameter of theinnermost groove, along with the degree of oxidation of the siliconsurface 12, ultimately determines the maximum exposed area of thepressure-sensitive membrane formed by the reflector 30. An innermostgroove having a larger diameter will enable a pressure membrane oflarger surface area to be formed. Accordingly, the sensitivity of thepressure sensor 10 may be increased. Conversely, an innermost groovehaving a smaller diameter will enable a pressure membrane of smallersurface area to be formed, resulting in a less sensitive pressure sensor10. Of course, the diameter of the innermost groove must not exceed theexternal diameter of the optical fibre 40. Thus, the diameter of theinnermost concentric groove is required to be less than the externaldiameter of the optical fibre 40 and is ideally greater than the corediameter of the optical fibre 40 so that all of the light transmitted bythe core is coupled to the cavity.

Furthermore, the diameter of the outermost groove is preferably greaterthan the external diameter of the optical fibre 40. This makes the taskof forming the channel 22 in the silicon substrate 11 relatively simpleas an area of the substrate 11, having a diameter at least that of theexternal diameter of the optic fibre 40 can be etched for a period oftime known to be sufficient or more than sufficient for etching the fulldepth of the substrate 11. However, where the external diameter of theoptical fibre 40 is greater than the outer diameter of the ring 18 thechannel etching process would only result in the base of the silicondioxide ring being exposed on all three surfaces. Also, although notillustrated it is envisaged that subsequent finishing processesincluding but not limited to coating the etched surfaces of the silicondioxide ring within the channel are envisaged.

The degree of oxidation is chosen such that a layer of around 4-5 μm ofsilicon dioxide is formed over the silicon wafer. This layer 19 ofsilicon dioxide supports the reflector 30 during the subsequentfabrication steps. Thicker or thinner layers 19 of silicon dioxide mayof course be employed as desired.

Naturally, the width and separation of the silicon ridges 16 may betailored to suit. However, care should be taken to ensure that the baseof the trough 18 of the silicon substrate 11 is not staved of oxygenduring the oxidation process. Alternatively, rather than forming aseries of concentric grooves 15, a single continuous trough, having thesame inner and outer diameter may be formed in the surtace of thesilicon substrate 11 which is subsequently filled by coating the trough,for example, with spin-on-glass or sol gel, for example, to form thefibre supporting ring 18. Multiple coatings may be required to fill theetched trough. With this fabrication process a signature depressionaround the ring would be formed on backing the fluid.

It will, of course, be appreciated that the depth of the grooves 15 orsingle trough formed in the silicon substrate 11 depends upon thedesired cavity length and the degree of oxidation (or thickness of thespin-on-glass) employed. Naturally, the depth of the grooves 15 orsingle trough and the degree of oxidation is adjusted according to thedesired cavity length.

The micro sensor 50 illustrated in FIG. 6 is an alternative design ofpressure sensor having many of the features of the sensor 1 illustratedin FIG. 1. In particular, the sensor 50 is constructed from a siliconsubstrate and consists of a sensor body 2 having a Fabry-Perot cavity 3and a membrane 4 disposed at a first end 5 of the cavity 3. The opposedsecond end 6 of the cavity 3 is in communication with a channel 7adapted to receive an optical fibre 40. The sensor body 2 includes ashoulder 8 which encircles the second end 6 of the cavity 3 and whichabuts and supports the end of the optical fibre 40 inserted into thechannel 7. Unlike the sensor 1 illustrated in FIG. 1, the pressuremembrane 4 comprises a thin reflective film 69 disposed over a silicondioxide member 67. The silicon dioxide member 67 comprises asubstantially planar portion 45 and a perimeter wall 44 surrounding theplanar portion 45. The perimeter wall 44 is preferably curved to hamperthe occurrence of cracks and other stress features that might otherwisearise due to high stress concentrations at a sharp comer, e.g. had theperimeter wall 44 met the planar portion at right-angle.

As shown in FIG. 7, the pressure sensor 50 illustrated in FIG. 6 isfabricated from a silicon substrate 51, such as a silicon wafer,provided with a layer of silicon dioxide 52 on an upper surface 53. Thesilicon dioxide layer 52 may be formed on the silicon substrate 51 byconventional means, but is preferably formed by plasma enhanced chemicalvapour deposition (PECVD) or thermal oxidation of the silicon substrate51 which results in chemical bonding between the silicon substrate 51and the silicon dioxide layer 52. The silicon dioxide layer 52 must bethick enough to act as a mask when subsequently etching the siliconsubstrate 51. However, as the thickness of the silicon dioxide layer 52increases the time taken to form the silicon dioxide layer 51 alsoincreases. Accordingly, the silicon dioxide layer is preferably not muchthicker than that required for subsequent etching of the siliconsubstrate 51. For the depth of etching that is contemplated in thepresent example, a silicon dioxide layer 51 of around 10 μm is suitable.

The exposed upper surface of the silicon dioxide layer 52 is coveredwith a photoresist mask 54 that is lithographically patterned with aseries of concentric grooves 55 similar to those formed in the mask 10described above and illustrated in FIG. 2. The photoresist mask 54 ispreferably a greyscale mask patterned such that the innermost wall 56 ofthe innermost groove 55 a is preferably curved so as to create a curvedperimeter wall 44 in the resulting silicon dioxide member 67.

The structure of the photoresist mask 54 is transferred to the silicondioxide layer 52 by reactive ion etching, and the structure of thesilicon dioxide layer 52 is subsequently transferred to the uppersurface 53 of the silicon substrate 51 by deep reactive ion etching toresult in the structure as shown in FIG. 8. The grooves 59 formed in thesilicon substrate 51 are again similar to those described above andillustrated in FIG. 2. Indeed, the only significant difference betweenthe silicon substrate 54 illustrated in FIG. 8 and that illustrated inFIG. 2 is the presence of a curved inner wall 57 for the innermostconcentric groove 59 a.

Any passivation layer deposited on the silicon substrate 54 during thedeep reactive ion etch is removed and the exposed upper surface 53 ofthe silicon substrate is oxidised in a manner similar to that describedabove and illustrated in FIG. 3. As shown in FIG. 9, the oxidationprocess results in a single ring or torus 61 of silicon dioxide whichfunctions as a fibre supporting structure for the sensor 50. Inaddition, a layer of silicon dioxide 60 is formed over the top surfaceof the silicon substrate 54. As in the fabrication of the sensorillustrated in FIG. 1, the silicon-silicon dioxide interface may not beentirely planar along the base of the ring 61 but may instead be rippledSimilarly, the upper surface of the ring 61 may have a correspondingripple. Again, the rippling is exaggerated in FIGS. 6 and 9 for ease ofidentification.

Following oxidation, a channel 63 generally cylindrical in shape andhaving a diameter sufficient to receive an optical fibre is etched,preferably by DRIE, into the opposed second surface 64 of the siliconsubstrate 54. The channel 63 extends into the silicon substrate 54 asfar as the silicon dioxide ring 61 which acts as an etch stop. Etchingcontinues to create a cavity 65 that is in communication with thechannel 63 and is defined by the innermost wall 66 of the silicondioxide ring 61 and the region 67 of silicon dioxide layer 60encompassed by the ring 61. The longitudinal axis of the channel 60 issubstantially normal to the inner surface of the silicon dioxide region67 encompassed by the ring 61. The diameter of the channel 63 ispreferably no greater than the outermost diameter of the silicon dioxidering 61.

After the channel 63 and cavity 65 have been formed, a thin reflectivefilm 69 is deposited over at least the inner surface 65 of the silicondioxide member 67 enclosed by the silicon dioxide ring 61. The film 69may be deposited using conventional deposition techniques, e.g. physicalor chemical vapour deposition. The reflective film 69 is preferablymetallic, such as aluminium or copper, having a thickness of a fewhundred Angstroms. However, as noted above for the pressure sensorillustrated in FIG. 1, the choice of material and thickness of the filmwill depend upon the desired pressure range and sensitivity of thesensor, as well as the choice of illuminating radiation. In particular,the film 69 may alternatively comprise a multilayer dielectric stack ora dichroic dielectric.

FIG. 10 illustrates a third embodiment of a pressure sensor 70. Thesensor 70 comprises a ring or torus 71 of silicon dioxide and a pressuremembrane 72 covering the opening at a first end of the ring 71. Anoptical fibre 73 is secured, by an adhesive or solder, to the ring 71 soas to cover the opening at the second opposed end of the ring 71 andthereby define a Fabry-Perot cavity 74. The pressure membrane 72comprises a layer of silicon dioxide 75 and a thin reflective film 76.

The reflective film 76 is preferably disposed on that surface 77 of thelayer of silicon dioxide 75 adjacent the Fabry-Perot cavity 74, but mayalternatively be disposed over that surface 78 remote from the cavity74. As in the case of the pressure sensors 1, 50 described above andillustrated in FIG. 1 and 6, the reflective film 76 is preferablymetallic, such as aluminium or copper, having a thickness of a fewhundred Angstroms. Alternative materials, such as a multilayerdielectric stack or a dichroic dielectric, may however be used accordingto the desired pressure range, sensitivity of the sensor and choice ofilluminating radiation.

The inner diameter of the ring 71 is preferably greater than thediameter of the core 79 of the optical fibre 73 so that all of the lighttransmitted by the optical fibre is coupled to the cavity 74.

As shown in FIG. 11, the pressure sensor 70 is fabricated from a siliconsubstrate 80, such as a silicon wafer, provided with a layer of silicondioxide 81 on an upper surface 82. As described above in the manufactureof the sensor 50 illustrated in FIG. 6, the silicon dioxide layer 81 maybe formed on the silicon substrate 80 by conventional means, and ispreferably formed by PECVD or thermal oxidation of the silicon substrate80. As before, a silicon dioxide layer 81 of around 10 μm thick issuitable.

The exposed upper surface of the silicon dioxide layer 81 is coveredwith a photoresist mask 83 that is lithographically patterned with acentral circular hollow 84 and a series of concentric grooves 85 havingprofiles and dimensions similar to those formed in the mask 10 describedabove and illustrated in FIG. 2.

The structure of the photoresist mask 83 is transferred to the silicondioxide layer 81 by reactive ion etching, and the structure of thesilicon dioxide layer 81 is subsequently transferred to the uppersurface 82 of the silicon substrate 80 by deep reactive ion etching toresult in the structure illustrated in FIG. 12. The grooves 86 formed inthe silicon substrate 80 have similar profiles and dimensions to thosegrooves 13 formed in silicon substrate 11 of the pressure sensor 1described above and illustrated in FIG. 2. The silicon substrate 80 ispreferably etched such that the central depression 87 has a depth thatis greater than that of the grooves 86. Nevertheless, the depression 87may alternatively have the same or smaller depth than that of thegrooves 86. The depression 87 should, however, be deep enough to ensurethat the pressure membrane 72 formed subsequently does not projectbeyond the top surface 90 of the silicon dioxide ring 71. As is furtherdiscussed below, the central depression 87 ultimately defines the shapeand size of the pressure membrane 72 of the sensor 80. The sensitivityof the pressure membrane 72 may therefore be adjusted by varying theshape and size of the central depression 87. In particular, byincreasing the diameter of the central depression 87 the sensitivity ofthe pressure membrane may be increased. Preferably, the diameter of thecentral depression 87 is greater than the diameter of the core 79 of theoptical fibre 73, but less than the outer diameter of the fibre 73, withwhich the sensor 80 is intended to be used.

Any passivation layer deposited on the silicon substrate 80 during thedeep reactive ion etch is removed and the upper surface of the siliconsubstrate is oxidised in a manner similar to that described above andillustrated in FIG. 3. As illustrated in FIG. 13, the oxidation processresults in a single ring or torus 71 of silicon dioxide which functionsas a fibre supporting structure for the sensor 70. In addition, a layerof silicon dioxide 75 is formed over the top of the depression in thesilicon substrate 80. This circular disc 75 of silicon dioxide is bondedto and is integral with the base of the ring 71. Again, as already notedabove, the top 90 and bottom 91 surfaces of the silicon dioxide ring 71may not be entirely planar but may instead be rippled.

The circular disc 75 of silicon dioxide forms part of the pressuremembrane 72 of the sensor 70. As already noted, the size and shape ofthe disc 75 will affect the sensitivity of the pressure membrane 72.Additionally, the thickness of the disc 75 will also affect thesensitivity of the membrane 72. After oxidation, the upper exposedsurfaces of silicon dioxide are etched using reactive ion etching untilthe disc 75 has a desired thickness.

A thin reflective film 76 is subsequently deposited over all exposedsilicon dioxide surfaces using conventional deposition techniques suchas physical or chemical vapour deposition. As noted above, thereflective film 76 is preferably metallic, such as aluminium or copper,having a thickness of a few hundred Angstroms, but alternativematerials, such as a multilayer dielectric stack or a dichroicdielectric, may also be used.

The thin reflective film 76 is then selectively etched from all silicondioxide surfaces with the exception of the inner wall 93 of the silicondioxide ring 71 and the upper exposed surface 94 of the silicon dioxidedisc 75.

An optical fibre 73 is then secured to the upper exposed surface 90 ofthe silicon dioxide ring 71 such that the core 79 of the fibre 73 isoptically coupled with the cavity 74 that is created between the opticalfibre 73 and the thin reflective film 76 disposed over the silicondioxide disc 75. The fibre 73 is secured to the ring 71 by conventionalmeans such as epoxy, or metal or glass soldering.

Finally, the silicon substrate 80 is removed using KOH or similaretchant to leave the fibre 73 and sensor 70 as depicted in FIG. 10.Alternatively, rather than removing the entire silicon substrate 80, achannel 95 may be etched back in the lower surface 96 of the siliconsubstrate 80, as shown in FIG. 14. The channel 95 is of sufficient sizeand shape to ensure that the entire pressure membrane 72 is exposed. Thechannel is preferably formed prior to the step of securing the opticalfibre 73 to the silicon dioxide ring 71 and may be etched usingconventional etching methods and preferably DRIE.

Whilst in the embodiment described above, the reflective film 76 isdeposited over that surface of the disc 75 adjacent the Fabry-Perotcavity 74, the reflective film 76 may alternatively be deposited overthat surface of the disc 71 remote from the cavity 74. For example,after the channel 95 has been etched, the silicon dioxide exposed by thechannel 95 may be etched by RIE to achieve a desired thickness of disc75. The thin reflective film 76 may then be deposited over that surfaceof disc 75 exposed by the channel 95.The micro sensor 100 illustrated inFIG. 15 is an alternative sensor adapted to measure the environmentaltemperature to which the sensor 100 is exposed. The sensor 100 isconstructed from a silicon-on-insulator (SOI) wafer and consists of asensor body having a channel 113 for receiving an optical fibre 40. Withthe optical fibre 40 in position within the channel 113, the opticalfibre 40 is optically coupled by means of a silicon dioxide layer 102 toa series of silicon dioxide ridges 108. The silicon dioxide ridges 108are coated with a luminescent material 110 the emission characteristicsof which are temperature dependent. In this example, the sensor 100 isadapted to receive an optical fibre having an external diameter of 200μm and a core diameter of 80 μm. As with the previous example, it willbe appreciated that the fabrication steps described herein, similar tothose described above, may be employed to manufacture a sensor adaptedto receive any size of optical fibre.

FIGS. 16 and 17 illustrate the steps employed in fabricating thetemperature sensor. With this method a sensor is fabricated which isadapted to luminesce in response to incident radiation delivered via anoptical fibre. The luminescent radiation can then be used to determineenvironmental conditions to which the sensor is exposed, such astemperature, fluid flow, pH, oxygen concentration, carbon dioxideconcentration, glucose concentration, lactate concentration, bicarbonateion concentration, chlorine ion concentration, sodium and potassium ionconcentration, etc.

As shown in FIG. 16, the sensor is manufactured from SOI i.e. a siliconsubstrate 101 having a first layer of silicon dioxide 102 and a seconduppermost layer of silicon 103 disposed thereon. The silicon dioxidelayer 102 is typically achieved by fusion bonding two oxide coatedsilicon wafers. The uppermost layer of silicon 103 is approximately 300μm thick, whilst the layer of silicon dioxide 102 is approximately 2 μmthick.

Concentric continuous grooves are formed on the surface 107 of thesilicon layer 103 about a common central axis X (three grooves areillustrated). The grooves extend the entire depth of the silicon layer103 such that regions 104 of the silicon dioxide layer 102 are revealedat the base of each groove. The sidewalls of each groove aresubstantially straight and each groove has a radial width of around 10μm and is separated from each neighbouring groove by a radial distanceof approximately 5 μm. Concentric ridges 105 of silicon are formedbetween each pair of neighbouring grooves, each concentric ridge 105having a generally rectangular profile with a height of 300 μm and aradial width of 5 μm. A central column 106 is also formed having adiameter of 5 μm.

The grooves are formed by applying a photoresist mask (not illustrated)over the surface 107 of the silicon layer 103 and etching the exposedsurfaces using deep reactive ion etching (DRIE). The silicon dioxidelayer 102 acts as the etch stop. As in the previous embodiment, othermasks and etching techniques may be employed to form the concentricgrooves.

Once the grooves have been formed, the etch maask is removed and thesilicon layer 103 is subjected to oxidation. The oxidation process ismaintained until all the silicon in the concentric silicon ridges 105has been oxidised. Preferably, wet thermal oxidation is employed owingto the high degree of oxidation that is achieved as well as therelatively fast rates of oxidation. Alternative forms of oxidation, suchas wet anodisation, chemical vapour deposition or plasma oxidation, maybe employed. However, their use would depend upon the wavelength of theemitted radiation. For example, for longer wavelengths, i.e. infra red,silicon could act as the core of the waveguide. Additionally, DRIE couldproduce waveguide structures in glass wafers anodically bonded to asilicon substrate.

The resulting structure, as shown in FIG. 17, comprises a siliconsubstrate 101 having a layer of silicon dioxide 102 disposed thereonwith a plurality (two are illustrated) of concentric silicon dioxideridges 108 extending substantially normal to the layer of silicon 101and a central column 106. On top of the silicon dioxide layer 102, tothe outside of the outermost concentric ridge 108, a further layer ofsilicon 103 is provided which has as layer of silicon dioxide 109 onwhat would otherwise be exposed surfaces of the silicon. To maximise theexposed surface area of the ridges 108, the separation of the individualridges of silicon 105 must be sufficient to prevent occlusion during theoxidation stage.

The silicon dioxide ridges 108 are thereafter coated with a luminescentmaterial 110 (FIG. 15). The choice of luminescent material and thethickness of the coating 110 will depend upon (a) the environmentalparameter to be measured (b) the method by which the parameter is to bemeasured and (c) the incident radiation used for excitation. Theluminescent coating 110 is preferably applied as a thin film so as tomaximise the sensitivity and response of the coating 110 to changes inthe environmental parameter to be measured. Accordingly, the luminescentmaterial is preferably applied using chemical vapour deposition.However, alternative forms of deposition may also be used, e.g. thermalevaporation, electron beam evaporation, sputtering, ink-jet,electrospraying, sol gel techniques etc. To optimise performance laserannealing and thermal annealing may be used to incorporate theluminescent ions in the host lattice of the luminescent material.

Depending upon the luminescent material used, changes in anenvironmental parameter may cause any of the following to occur: (1) achange in the peak intensity of the luminescent radiation, (2) a shiftin the wavelength of the luminescent radiation, (3) a change in decaytimes of the luminescent radiation. Examples of luminescent materialssuitable for measuring a whole range of environmental parameters can befound, for example, in “Ruthenium complex entrapped in a porous sol gelfilm”, A. K. McEvoy et al, SPIE, vol.2508, pp 190-198; “The Oxylite: Afibre-optic oxygen sensor”, J. R. Griffiths et al., Brit. J. Radiology,vol. 72, pp 627-630, 1999; “Development of a Medical Fiber-Optic OxygenSensor Based on Optical Absorption Change”, R. A. Wolthuis et al., IEEETrans Biomed. Eng., vol. 39, pp 185-193, 1992; “Fiber-optic oxygensensor using molybdenum chloride cluster luminescence”, R. N. Ghosh etal., App. Phys. Lett., vol. 75, pp 2885-2887, 1999; “Sol gel based fiberoptic pH sensor”, S. A. Grant et al., SPIE, vol. 2976, pp 64-70; “WideRange pH Fiber Sensor with Congo-Red and Methyl-Red Doped Poly (MethylMethacrylate) Cladding”, Egami et al., Jap. J. App. Phys., vol.36,pp-2902-2905, 1997; “Gastric pH sensing with CPGs fixed at the distalend of plastic optical fibres”, F. Baldini et al., SPIE, vol.2293, pp149-153; “Absorbance-based affinity glucose sensor”, S. Mansouri et al.,Optical Fibers in Medicine, SPIE, vol. 906, pp 57-59, 1988; “Advances infibre-optic sensors for in-vivo monitoring”, F. Baldini et al., SPIE,vol. 2508, pp 117-135; “Ion-selective sensing based on potentialsensitive dyes”, Z. Zhujun et al., Optical Fibers in Medicine III, SPIE,vol. 906, pp. 74-79, 1988.

Preferably the luminescent coating 110 is then annealed to improve itsperformance. UV laser annealing is preferred owing to its ability todeliver high temperatures (ca. 1800 K) into an absorbing surface up to 1μm in thickness. Alternatively, the sensor may be globally annealed, sayat 1000 K, over a period of several hours. When global annealing isemployed, the sensor may be annealed in an oxygen rich atmosphere tofurther improve the performance of luminescent coatings havingoxide-based host lattices, e.g. yttrium oxide.

Following annealing, a channel 113 is formed in the opposed secondsurface of the silicon substrate 101. The channel 113 is generallycylindrical in shape and has a diameter sufficient to accommodate anoptical fibre. The channel 113 extends the entire length of the siliconsubstrate 101 and terminates at the silicon dioxide layer 102. Thelongitudinal axis of the channel 113 is generally parallel andcoincident with the longitudinal axis X of the concentric ridges 108.Preferably the maxinum diameter of the ridges 108 is less than the outerdiameter of the optical fibre such that a peripheral supportingstructure in the form of the silicon dioxide layer 102 and the uppersilicon layer 103 is provided to support the edges of the optical fibre.

The channel 113 is preferably formed using DRIE owing to the relativelyhigh etch rates and the ability to achieve substantially verticalsidewalls. Similar to the previous example, the silicon dioxide layer102 acts as the etch stop. Alternative forms of isotropic etching maynevertheless be employed.

With reference to FIG. 15, it can be seen that good optical coupling canbe achieved between the core 42 of the optical fibre 40 and theluminescent material 110 via the silicon dioxide layer 102 and ridges108 which acts as an optical coupling between the optical fibre and theluminescent material 110. The ridges 108 serve to collect theluminescence radiation and channel it towards the optical fibre 40.Furthermore, by using ridges 108 the area of luminescent material 110 ismaximised to increase the signal intensity.

The wavelength of the incident radiation may be chosen so as to increasethe amount of collected radiation emitted by the luminescent material110. For example, incident radiation of wavelength X may cause theluminescent material 110 to emit radiation of wavelength Y. Therefractive index of the luminescent material 110 and the silicon dioxideridges 108 at wavelength Y might be 1.3 and 1.445 respectively. Thiscorresponds to a cone of half angle of 64°. Accordingly, any lightemitted by the luminescent material 110 which meets the silicon dioxideinterface at an angle within this cone will pass through the silicondioxide ridges 108 due to critical angle considerations. Thiscorresponds to roughly 70% of the emitted radiation being lost. If,however, the incident radiation is chosen such that emitted radiationhas a different wavelength, and the refractive index of the luminescentmaterial 110 at this wavelength is much lower, say approaching 1, thenthe cone of half angle is reduced to 43°. Accordingly, less of theradiation emitted by the luminescent material 110 is lost.Alternatively, an increase in the amount of collected luminescentradiation may be achieved by employing luminescent materials 110 havinga low index of refraction for the wavelength emitted, or where there isa large step in the indices of refraction for the luminescent material 110 and silicon dioxide ridges 108 at the emitted wavelength.

Before the silicon dioxide ridges 108 are coated with luminescentmaterial, the ridges may first be coated with a cladding material, forexample MgF₂, doped or porous silica or some other dielectric, whichserves to protect the silicon dioxide ridges. Moreover, a claddingmaterial having a refractive index between that of the silicon dioxideridges and the luminescent material would aid in channelling theluminescent radiation along the ridges towards to the optical fibre,i.e. the cladding would act as an antireflective coating for theradiation emitted by the luminescent material. The cladding layer may beapplied using conventional deposition techniques, e.g. sputter, chemicalvapour deposition (PECVD or LPCVD) or evaporation.

Although in the above example silicon dioxide ridges 108 in the form ofconcentric rings are described, other structures are possible whilstgreatly increasing the available surface area. For example, rather thanforming a series of concentric grooves in the surface 107 of the siliconlayer 103, a series of parallel, linear grooves may alternatively beformed. This would then result in a series of planar ridges upon whichto deposit the luminescent material. A further alternative that isenvisaged is a plurality of freestanding cylindrical columns, arrangedfor example in a hexagonal array. Where the luminescent material isapplied as an ink, or other liquid, freestanding columns encourages flowof the material about and between the silicon dioxide ridges. It will beappreciated that the number, shape and size of the silicon dioxidestructures may be tailored to suit. Moreover, the surface area of thestructures may be increased by increasing the thickness of the siliconlayer 103.

The grooves formed on surface 107 of the silicon layer 103 preferablyhave an open profile such that an obtuse angle is formed by thesidewalls and the base of each groove. In having a groove with an openprofile, the oxide ridges 108 may promote waveguiding for luminescentmaterials having a lower refractive index than that of the ridges 108.

Owing to the relatively high surface area of the luminescent coating andthe high optical coupling between the optical fibre and the luminescentcoating, sufficient luminescent intensity can be collected even fromthin luminescent coatings. Accordingly, sensors having a quick responsetime (sub-minute) are possible. In addition, luminescent materialshaving a fast decay time may be used. Generally, fast decay times areonly possible at the expense of luminescent intensity. Sensors having ashorter dead time may therefore be formed.

FIG. 18 illustrates an alternative design of luminescent sensor 120manufactured from a silicon rather than SOI wafer. As with the sensor100 described above and illustrated in FIG. 15, the sensor 120 comprisesa sensor body 121 having a channel 113 for receiving an optical fibre40. With the optical fibre 40 in position within the channel 113, theoptical fibre 40 is optically coupled to a series of silicon dioxideridges 108, which are coated with a luminescent material 110.

The ridges 108 are preferably tapered so as to yield a high absorptioncross-section per unit length. For ridges 108 having a generallyrectangular profile, the absorption of the incident radiation by theluminescent material 110 is not uniform along the length of the ridge108 but is generally exponential. Accordingly, only a small length ofluminescent material 110 is effectively used. The profiles of the ridges108 are therefore preferably tapered. Moreover, the profile ispreferably parabolic, polynomial or exponential so as to achieve aquasi-linear absorption coefficient along the length of the ridges 108.

Each silicon dioxide ridge 108 preferably has a width at its base ofbetween 2 and 10 μm, tapering to a width at its tip of between 0.2 and 5μm, and a height of up to 500 μm. In having silicon dioxide ridges 108of a width comparable, or indeed smaller, than the wavelength ofincident radiation, good optical coupling is achieved with theluminescent material 110 surrounding the ridges 108. Nevertheless, itwill be understood that alternative dimensions for the ridges 108 may beemployed to suit.

The height of each ridge 108 may be longer than necessary so as toaccount for process tolerances, though this may result in some reductionin sensitivity. For example, the height of a ridge 108 having a width atits base of 10 μm and an exponential taper is around 150 μm However, inhaving a ridge height of say 200 μm fabrication tolerances can beaccounted for.

By suitable choice of taper, again at some expense of sensitivity, theeffective coupling region of absorption can be moved up and down thelength of the ridges 108. Accordingly, the effective coupling region canbe moved away from inconsistent artifacts in the ridges that arise fromthe fabrication process. In particular, excess luminescent material 110may collect at the base of the ridges 108. The increased thickness ofthe luminescent material 110 at the base of the ridges 108 may be suchthat no penetration of the analyte can occur and therefore the region iseffectively useless. Similarly, the thickness of the luminescentmaterial may be ill-defined at the very tip of the tapered ridge 108.Through a suitable choice of taper, the effective coupling region may bemoved to a region uniform, well-defined, reproducible thickness ofluminescent material 110.

Additionally, by employing tapered ridges 108, single mode operation ofthe optical fibre 40 and sensor 120 is made possible due to theefficient coupling that is obtained. Single mode operation isparticularly desirable as the incident radiation used to excite theluminescent material 110 may be a top-hat light source in which higherorder modes exhibit different intensities due to environmentalconditions such as fibre flexing, which could result in unpredictablesensor operation. Additionally, modelling results suggest that differentmodes have widely different absorption coefficients, which again couldresult in unpredictable sensor behaviour.

As shown in FIG. 19, the luminescent sensor 120 is fabricated from asilicon substrate 121, such as a silicon wafer, provided with a layer ofsilicon dioxide 122 on an upper surface 123. The silicon dioxide layer122 may be formed on the silicon substrate 121 by conventional means,and is preferably formed by PECVD or thermal oxidation of the siliconsubstrate 121.

The exposed upper surface of the silicon dioxide layer 122 is coveredwith a greyscale photoresist mask 124, such as chrome-on-glass or HEBSmask The mask 124 is lithographically patterned with a series ofconcertric grooves 125, 126. The profiles of the grooves 125 formed in acentral region of the mask 124 are tapered such that the mask 124comprises a series of concentric tapered ridges 127. The grooves 126surrounding the tapered ridges 127 have conventional rectangularprofiles. Owing to mask 124 having different shaped grooves 125,126, thesubsequent etching of the tapered grooves in the silicon substrate 121will generally proceed at different rate to that of the rectangulargrooves. In order that the subsequent rectangular and tapered groovesformed in the silicon substrate 121 have the same depth, the Angulargrooves 126 in the photorsist mask 124 preferably do not extend throughthe entire mask 124. Instead, the grooves 126 stop short of the silicondioxide layer 122 to leave a compensation layer. The thickness of thecompensation layer will naturally depend, among other things, on thedepth of the grooves and ridges to be etched in the silicon substrate121.

The shape, size and number of tapered ridges 127 formed in the mask 124may be tailored to suit As noted above, the profile of the taperedridges 127 is preferably parabolic, polynomial or exponential.

The structure of the greyscale photoresist mask 124 is transferred tothe silicon dioxide layer 122 by reactive ion etching, and the structureof the silicon dioxide layer 122 is subsequently trasferred to the uppersurface 123 of the silicon substrate 121 by deep reactive ion etching toresult in the structure illustrated in FIG. 20. By way of example, eachof the grooves 128 formed in the silicon substrate 121 and having asubstantially rectangular profile has a depth of around 300 μm and aradial width of 5 μm The separation between neighbouring rectangulargrooves 128 is about 5 μm Each tapered ridge 129 formed in the siliconsubstrate 121 also has a height of around 300 μm and a radial width atthe base of around 5 μm. Preferably, there is no separation betweenneighbouring tapered ridges 129 at their base. The tapered ridges 129preferably cover E region greater than the core diameter of the opticalfibre 40 so that all of the light transmitted by the core 42 is coupledto the subsequent silicon dioxide ridges 108. It will of course beappreciated that the dimensions indicated above are provided by way ofexample and that the size and shape of the grooves 128 and ridges 129formed in the silicon substrate 121 may be tailored to suit In theexample provided above, the depth of the rectangular grooves 128 and theheight of the tapered ridges 129 are substantially similar. However, theheight of the ridges 129 may equally be greater and smaller than thedepth of the grooves 128.

As detailed above, the height of each ridge 129 is preferably longerthan necessary so as to account for fabrication tolerances.

Any passivation layer deposited on the silicon substrate 121 during thedeep reactive ion etch is removed and the upper surface of the siliconsubstrate is oxidised in a manner similar to that described above andillustrated in FIG. 3. As illustrated in FIG. 19, the oxidation processresults in a circular disc 130 of silicon dioxide. In addition, a layerof silicon dioxide 131 is formed over the top surface 123 of the siliconsubstrate 121.

The lower surface 132 of the silicon dioxide disc 130 may not beentirely planar but may instead be rippled due to the oxidation process.The upper surface of the circular disc 130 comprises a series ofconcentric ridges 108 encompassed by ring 133 which again may berippled.

The silicon dioxide ridges 108 are thereafter coated with a luminescentmaterial 110. As noted above in the fabrication of the sensorillustrated in FIG. 15, the choice of luminescent material and thethickness of the coating 110 will depend upon (a) the environmentalparameter to be measured (b) the method by which the parameter is to bemeasured and (c) the incident radiation used for excitation. Theluminescent coating 110 is preferably applied as a thin film so as tomaximise the sensitivity and response of the coating 110 to changes inthe environmental parameter to be measured. The luminescent material 110is preferably applied using chemical vapour deposition, but alternativeforms of deposition may also be used, including thermal evaporation,electron beam evaporation, sputtering, ink-jet, electrospraying, and solgel techniques.

As in the case of the luminescent sensor described above and illustratedin FIG. 15, the silicon dioxide ridges 108 may first be coated with acladding or antireflective coating (not shown) prior to coating theridges 108 with the luminescent material 110. Additionally, theluminescent coating 110 may be annealed to improve performance.

A channel 1 13 is then formed in the opposed second surface 134 of thesilicon substrate 121. The channel 113 is generally cylindrical in shapeand has a diameter sufficient to accommodate an optical fibre. Thechannel 113 extends the entire length of the silicon substrate 121 andterminates at the silicon dioxide disc 130. The longitudinal axis of thechannel 113 is generally parallel and coincident with the longitudinalaxis of the concentric ridges 108.

The channel 113 is preferably formed using DRIE owing to the relativelyhigh etch rates and the ability to achieve substantially verticalsidewalls.

As illustrated in FIG. 19, good optical coupling is achieved between thecore 42 of the optical fibre 40 and the luminescent material 110surrounding the silicon dioxide ridges 108 via the silicon dioxide disc130 which acts as an optical coupling between the optical fibre and theluminescent material 110.

Although in the example describe above, the silicon dioxide ridges 108are in the form of concentric rings, other structures are againpossible, including, but not limited to, a series of planar ridges or aplurality of freestanding columns.

Thus far, the manufacture of two different types of sensor has beendescribed. The first sensor, a pressure sensor, was manufactured from asilicon wafer whilst the second sensor, or luminescent sensor, wasmanufactured from either a silicon or an SOI wafer. It will beimmediately apparent that the pressure sensor and the luminescentsensor, when fabricated from a silicon wafer, may be manufactured from asingle silicon wafer. However, both types of sensor may be also bemanufactured from a single SOI wafer. Indeed, with the fabricationmethods described above several separate sensors for measuring differentenvironmental parameters may be constructed on a single wafer.

When fabricating both types of sensor on a single SOI wafer, thepressure sensor is first manufactured by applying a photoresist mask, orsimilar, over the SOI wafer and etching through the uppermost siliconlayer and silicon dioxide layer to expose a region of the lower siliconsubstrate layer. The manufacture of the pressure sensor then continueson the exposed area of the silicon substrate as described above. Thatis, a ring of silicon dioxide is formed in the bulk silicon.

Attention now turns to the manufacture of the luminescent sensor.Following the oxidation of the wafer in forming the pressure sensor, alayer of silicon dioxide now coats the uppermost layer of silicon.Nevertheless, the process for forming the luminescent sensor remainspretty much the same. The only significant difference is that theconcentric grooves are now formed on the surface of the silicon dioxidelayer and extend the entire depth of both the silicon dioxide layer andthe silicon layer. The wafer is then again oxidised to form the silicondioxide ridges which act to collect and channel the luminescentradiation. Alternatively, the first oxidation step in the manufacture ofthe pressure sensor may be omitted. In this situation, the two oxidationsteps are replaced by a single oxidation process which occurs after alletching of both the pressure sensor and luminescence sensor has takenplace to form two separate silicon dioxide regions in the wafer.

Luminescent materials are then selectively deposited using a photoresistlift-off process or ink dropper process, after which the reflectivelayer of the pressure sensor is defined. This sequence, however, isdependent upon the nature of the materials and the annealing processesused in the fabrication of the sensors. Should cladding of the silicondioxide ridges be required, a coating of cladding material isselectively deposited prior to the application of the luminescentcoating.

The wafer is then annealed to improve the performance of the luminescentmaterial. As mentioned before, UV laser annealing is preferred so thatthe annealing may be localised. However, where this will not adverselyaffect other sensors being fabricated on the same wafer, the wafer maybe instead be globally annealed.

Finally, individual channels for receiving optical fibres for eachsensor are etched on the reverse side of the wafer and are aligned withthe individual sensors and their respective silicon dioxide regions.

In addition to the sensors described above, an imaging sensor may alsobe fabricated in the SOI wafer. The imaging sensor permits radiationexternal to the sensor to be collected by the sensor and communicatedback along an optical fibre. In addition, the imaging sensor may be usedto illuminate the region around the sensor and collect images of thatregion.

The imaging sensor is manufactured by applying a photoresist mask, orsimilar, over the SOI wafer and etching through the uppermost siliconlayer and the silicon dioxide layer to expose a substantially circularregion of the lower silicon substrate layer. The etch is preferablyisotropic such that the walls of the etched region form an obtuse anglewith the exposed surface of the silicon substrate layer. The size of thephotoresist mask is chosen such that the diameter of the exposed regionof the silicon substrate surface is less than the external diameter ofthe optical fibre to be received and preferably larger than the corediameter of the optical fibre. A channel is then etched on the reverseside of the SOI wafer. The channel is generally cylindrical in shape andhas a diameter sufficient to accommodate an optical fibre. The channelextends the entire length of the lower silicon substrate and terminatesat the silicon dioxide layer. The centre of the channel generallycorresponds with the centre of the etched region formed in the uppermostsilicon layer and silicon dioxide layer of the wafer. Thus, the aperturein the silicon dioxide layer functions as an optical coupling between anoptical fibre located within the channel and the region beyond the uppersurface of the wafer.

Whilst reference has been made to etching the uppermost layers of theSOI wafer to expose a circular region of the silicon substrate layer, itwill be appreciated that the exposed region may be of any shape and sizeso long as the optic fibre is prevented from passing entirely throughthe sensor when inserted into the channel.

Alternatively, the imaging sensor may be formed in the same manner asthat employed for forming the pressure sensor. However, in forming theimaging sensor, the step of applying a reflector is omitted.

Whilst each sensor formed on the SOI wafer has thus far been describedas having its own channel etched on the reverse side of the SOI waferfor receiving an optical fibre, a single channel may instead be etchedon the reverse side of the SOI wafer for all sensors The single channelextends the entire length of the lower silicon substrate and terminatesat the silicon dioxide layer of each sensor. The single channel has adiameter sufficient to receive a single optical fibre having a corediameter suitable for optically coupling with each sensor. It will ofcourse be appreciated that with this embodiment a multi-mode opticalfibre is preferred.

Accordingly, a plurality of sensors maybe formed on a common substrate.The sensors may be connected to a plurality or bundle of optical fibres,each optical fibre being associated with one sensor, or the sensors maybe connected to single optical fibre. In either case, the optical fibremay be single or multi-mode.

The most common method of securing an optical fibre within a channelformed in a silicon substrate is to apply a ring of adhesive about thefibre which bonds with the surface of the silicon substrate and thefibre. However, repeated small displacements of the optical fibre withrespect to the substrate over time can cause the adhesive to becomeseparated from the substrate surface.

With reference to FIG. 21, in order to better secure the fibre 40 to thesilicon substrate 101, an annular groove 111 may be formed in thesurface 12′ of the silicon substrate or silicon block 101 adjacent theopening of the channel 22. When the adhesive 112 (e.g. UV curableadhesive) is applied to the substrate 101 to secure the fibre 40, theadhesive 112 fills the annular groove 111. When cured, the adhesive 112in the annular groove 111 further inhibits separation of the adhesive112 from the substrate surface 12′ due to displacements parallel to thesurface 12′ of the silicon substrate 101. Furthermore, the width of thegroove at the surface 12′ of the substrate 101 is preferably less thanthe width of the groove below the surface 12′ of the substrate such thatdisplacements perpendicular to the substrate surface 12′ are alsofurther inhibited.

Whilst the groove 111 is preferably annular with a closed profile, itwill be appreciated that other shaped grooves having alternativeprofiles may be employed Moreover, the same technical effect may beachieved with one or more notches formed in the silicon substrate 101 atpositions near the channel opening.

By employing the methods described above, several sensors may bemanufactured on a single wafer. Each sensor may be adapted to measure adifferent environmental parameter. For example, the wafer may compriseseveral luminescent sensors, each having a different luminescentmaterial for measuring parameters such as temperature, fluid flow, pH,oxygen concentration, carbon dioxide concentration, glucoseconcentration, lactate concentration, bicarbonate ion concentration,chlorine ion concentration, sodium and potassium ion concentration, etc.Different pressure sensors may be employed having different pressuremembranes and/or cavity lengths for measuring different pressure ranges.

It is anticipated that the sensors will have practical applications inthe field of medicine, particularly where sensors having quick(sub-minute) response times can be vital. With the present invention,sensors having a sub-millimetre diameter are possible making the sensorssuitable for mounting on the end of an in vivo probe, such as anendoscope.

The sensors may be used in in vivo patient monitoring, such as thatperformed in intensive care units, elective surgery and anaesthetics.The sensors may also be used in conjunction with drug delivery systemsarranged in feedback, such that drug delivery is controlled according tomeasurements made by the sensors. The sensors may also be used to assessparticular medical conditions. For example, metabolic and hormone levelscan be monitored using these sensors which is of use in assessingfertility levels. The sensors may also be of use in the field ofpsychiatry by measuring, for example, the levels of neurotransmitterssuch as serotonin.

It is possible that materials used to measure in vivo properties may betoxic. Accordingly, the sensor may be encapsulated by a container ofnon-toxic material having at least one valve, or closable aperture, forpermitting the ingress of an external fluid. When a measurement is to bemade, the valve is opened to receive the fluid. Preferably, thecontainer is evacuated and has a one-way inlet valve.

The sensors of the present invention are also suitable for use in foodprocessing. For example, the sensors may be used to probe foodstuffs tomeasure temperature and/or pH levels.

As the sensors are preferably constructed from monolithic silicon, theycan be manufactured using conventional techniques employed in MEMSmanufacture. This makes the sensors suitable for mass production.Accordingly, disposable, sterile sensors having low production costs maybe manufactured.

The sensor may also be used for a wide variety of applications includingbut not limited to air conditioning systems (measuring for exampletemperature, humidity, and CO2 levels), computerised engine tuningsystems for vehicles (measuring for example the temperature andcomposition of the exhaust), EMP and EMC test facilities.

1. A method of fabricating an optical sensor comprising the steps of:providing a silicon substrate having a first surface and a secondsurface; providing a region comprising essentially of silicon dioxide onor in the first surface of the silicon substrate; etching a channel intothe silicon substrate from said second surface up to said silicondioxide region, said channel being sized to receive an optical fibrewhereby said silicon dioxide region forms an end portion of said channelwhich at least partially closes said channel; and coating at least aportion of the silicon dioxide region with a coating to form anenvironmentally-sensitive element.
 2. A method of fabricating an opticalsensor as claimed in claim 1, wherein the silicon substrate and silicondioxide region form a single substrate element.
 3. A method offabricating an optical sensor as claimed in claim 1, wherein the siliconsubstrate is monolithic.
 4. A method of fabricating an optical sensor asclaimed in claim 1, wherein the step of providing said silicon dioxideregion comprises oxidising a portion of said first surface of saidsilicon substrate.
 5. A method of fabricating an optical sensor asclaimed in claim 1, wherein the step of providing said silicon dioxideregion comprises etching at least one continuous groove in the firstsurface of said silicon substrate and thereafter forming silicon dioxidein said at least one continuous groove.
 6. A method of fabricating anoptical sensor as claimed in claim 1, further comprising the step offorming at least one projection comprising essentially of silicondioxide on said silicon dioxide region.
 7. A method of fabricating anoptical sensor as claimed in claim 6, wherein the step of forming atleast one projection comprises providing a layer of silicon over saidsilicon dioxide region; etching the layer of the silicon to form atleast one structure projecting outwardly from said silicon dioxideregion; and thereafter oxidising the at least one structure to form saidat least one projection.
 8. A method of fabricating an optical sensor asclaimed in claim 6, wherein the step of forming at least one projectioncomprises etching the first surface of said silicon substrate to form atleast one structure projecting outwardly from the silicon substrate andthereafter oxidising at least a portion of the first surface of saidsilicon substrate including the projecting structure to form saidsilicon dioxide region and said at least one projection.
 9. A method offabricating an optical sensor as claimed in claim 7, wherein the step ofetching the silicon comprises etching at least two concentric grooves toform one or more continuous projecting walls.
 10. A method offabricating an optical sensor as claimed in claim 7, wherein the step ofetching the silicon comprises etching two or more linear parallelgrooves to form at least one planar projecting wall.
 11. A method offabricating an optical sensor as claimed in claim 7, wherein the step ofetching the silicon comprises etching a plurality of enclosed grooves toform a plurality of freestanding projections.
 12. A method offabricating an optical sensor as claimed in claim 6, wherein the profileof the silicon dioxide projection is tapered.
 13. A method offabricating an optical sensor as claimed in claim 1, wherein the step ofcoating at least a portion of the silicon dioxide region to form anenvironmentally-sensitive element comprises coating at least a portionof the silicon dioxide region with a luminescent material.
 14. A methodof fabricating an optical sensor as claimed in claim 1, wherein thesilicon dioxide region includes a shoulder to define a constriction atthe end portion of the channel and the step of coating at least aportion of the silicon dioxide region to form anenvironmentally-sensitive element comprises coating a region of silicondioxide that closes the constricted end of the channel with a reflectivematerial.
 15. A method of fabricating an optical sensor as claimed inclaim 14, further comprising the step of etching a region of silicondioxide that closes the constricted end of the channel after thereflective material has been applied.
 16. A method of manufacturing asensor comprising the steps of: providing a silicon wafer; forming atleast one continuous groove in a first surface of the silicon wafer;forming silicon dioxide in said continuous groove and over the surfacearea of the silicon wafer encompassed by the continuous groove to form alayer of silicon dioxide; forming a channel in the silicon waferextending from an opposed second surface of the silicon wafer up to thebase of the continuous groove containing silicon dioxide and forming acavity beyond the channel in which the wall of the cavity is defined bythe silicon dioxide provided in the continuous groove, the longitudinalaxis of the channel and the cavity substantially intersecting the pointof centre of the continuous groove; and applying a reflector over atleast the layer of silicon dioxide encompassed by the continuous groove.17. The method as claimed in claim 16, wherein the step of forming thecontinuous groove in the silicon wafer comprises applying a mask havinga ring aperture over the first surface of the silicon wafer and etchingexposed regions of the first surface to form an annular groove.
 18. Themethod as claimed in claim 16, wherein a plurality of concentriccontinuous grooves are formed in the first surface of the silicon wafer,each groove having a thickness and being separation from an adjacentgroove a distance selected such that oxidation of the first surface ofthe silicon wafer including the plurality of concentric grooves producesa silicon dioxide torus in the first surface of the silicon wafer. 19.The method as claimed in claim 16, wherein the step of applying areflector comprises coating at least the silicon dioxide layerencompassed by the continuous groove with a thin reflective film. 20.The method as claimed in claim 16, further comprising the step ofremoving at least the silicon dioxide layer encompassed by thecontinuous groove after the reflector has been applied.
 21. A method ofmanufacturing a sensor comprising the steps of: providing a siliconwafer having a first surface and a second opposed surface; forming adepression in the first surface of the silicon wafer; forming silicondioxide over the surface of the depression and at least a region of thesilicon surface surrounding the depression to form a silicon dioxidemembrane surrounded by a region of silicon dioxide; etching the opposedsecond surface of the silicon wafer up to the silicon dioxide to fullyexpose the silicon dioxide membrane; and coating the silicon dioxidemembrane with a reflector, wherein the silicon dioxide region serves asa fibre supporting structure for an end of an optical fibre.
 22. Amethod of manufacturing a sensor comprising the steps of: providing asilicon-silicon dioxide-silicon wafer; etching an upper surface of thesilicon-silicon dioxide-silicon wafer to form at least one siliconprojection; oxidising at least a portion of the exposed upper surface ofthe wafer including the silicon projection so as to form at least onesilicon dioxide projection; coating at least a portion of the silicondioxide projection with a luminescent material; and forming a channel inthe opposed surface of the silicon-silicon dioxide-silicon wafer as faras the silicon dioxide layer, the longitudinal axis of the channel beingsubstantially aligned with the silicon dioxide projection.
 23. A methodof manufacturing a sensor comprising the steps of: providing a siliconwafer having a first surface and a second opposed surface; etching thefirst surface of the silicon wafer to form at least one siliconprojection; oxidising at least a portion of the first surface of saidsilicon substrate including the silicon projection to form a silicondioxide layer and a silicon dioxide projection; coating at least aportion of the silicon dioxide projection with a luminescent material;and forming a channel in the second surface of the silicon wafer as faras the silicon dioxide layer, the longitudinal axis of the channel beingsubstantially aligned with the silicon dioxide projection.
 24. Themethod as claimed in claim 22, wherein the profile of the silicondioxide projection is tapered.
 25. A method of fabricating a pluralityof optical sensors on a common substrate comprising the steps of:providing a silicon substrate having a first surface and a secondsurface; providing at least two regions each comprising essentially ofsilicon dioxide on or in the first surface of the silicon substrate;coating at least a portion of each silicon dioxide region with a coatingto form an environmentally-sensitive element; and etching at least onechannel into the silicon substrate from said second surface up to one ormore of said silicon dioxide regions, the channel being sized to receivean optical fibre whereby said one or more silicon dioxide region formsan end portion of the channel which at least partially closes saidchannel.
 26. A method of fabricating a plurality of optical sensors asclaimed in claim 25, wherein the step of etching at least one channelcomprises etching a plurality of channels, each channel being sized toreceive an optical fibre and whereby each silicon dioxide region formsan end portion of the respective channel which at least partially closessaid channel.
 27. A method of fabricating a plurality of optical sensorsas claimed in claim 25, wherein the step of etching at least one channelcomprises etching a single channel optically coupled to eachenvironmentally-sensitive element.
 28. An optical sensor comprising: asilicon substrate having a first surface and an opposed second surface;a channel extending into the silicon substrate from said second surface,said channel being sized to receive an optical fibre and having an endportion distant from said second surface, said end portion at leastpartially closing said channel and comprising essentially of silicondioxide; and a coating disposed over at least a region of the silicondioxide to define an environmentally-sensitive element.
 29. An opticalsensor as claimed in claim 28, wherein the silicon substrate and silicondioxide forms a single substrate element.
 30. An optical sensor asclaimed in claim 28, wherein the silicon dioxide includes a shoulder todefine a constriction at the end portion of the channel and the coatingcomprises a reflective material that covers a region of silicon dioxidethat closes the constricted end of the channel.
 31. An optical sensor asclaimed in claim 28, wherein the silicon dioxide only partially closesthe channel to create an opening in the end portion of the channel andthe coating comprises a reflective material that covers a region ofsilicon dioxide surrounding the opening and extends over the opening.32. An optical sensor as claimed in claim 28, wherein said silicondioxide comprises at least one projection and the coating is aluminescent coating applied over at least a portion of the at least oneprojection.
 33. A sensor comprising a silicon wafer having a cavity in afirst surface covered by a reflector and a channel extending from anopposed second surface of the silicon wafer to the cavity and being incommunication therewith, the diameter of the channel being greater thanthe diameter of the cavity and the end of the channel adjacent thecavity comprising essentially of silicon dioxide.
 34. A sensor asclaimed in claim 33, wherein the reflector comprises a thin metal film.35. A sensor as claimed in claim 33, wherein the reflector comprises adielectric stack.
 36. A sensor comprising a silicon wafer having atleast a region of a first surface of the silicon wafer covered by alayer of silicon dioxide and at least one structure comprisingessentially of silicon dioxide projecting outwardly from the silicondioxide layer and having a luminescent material covering at least aportion of said silicon dioxide structure and a channel extending froman opposed second surface of the silicon wafer to said silicon dioxidelayer and aligned with said silicon dioxide structure.
 37. A sensor asclaimed in claim 36, wherein the profile of said silicon dioxidestructure is tapered.
 38. A sensor system comprising a plurality ofoptical sensors on a common substrate having a first surface and anopposing second surface and a channel extending into the commonsubstrate from said second surface, said channel being sized to receivean optical fibre, each optical sensor comprising: an end portion distantfrom said second surface at least partially closing said channel andcomprising essentially of silicon dioxide; an optical couplingassociated with said end portion; and at least one of said opticalsensors further comprising an environmentally-sensitive element foroptical coupling with an optical fibre by means of said opticalcoupling.
 39. A sensor system comprising a common substrate ofmonolithic silicon having a first surface and an opposing second surfaceand at least one pressure sensor and at least one optical sensor formeasuring a parameter selected from temperature, fluid flow, pH, oxygenconcentration, carbon dioxide concentration, glucose concentration,lactate concentration, bicarbonate ion concentration, chlorine ionconcentration, sodium and potassium ion concentration, the pressuresensor comprising: a cavity formed in the first surface of the substratecovered by a reflector and a channel extending from the second surfaceof the substrate to the cavity and being in communication therewith, thediameter of the channel being greater than the diameter of the cavityand the end of the channel adjacent the cavity comprising essentially ofsilicon dioxide; and the optical sensor comprising: a layer of silicondioxide covering at least a region of the first surface of the substrateand at least one structure comprising essentially of silicon dioxideprojecting outwardly from the silicon dioxide layer and having aluminescent material covering at least a portion of said silicon dioxidestructure and a channel extending from the second surface of thesubstrate to said silicon dioxide layer and aligned with said silicondioxide structure.
 40. A method of securing an optical fibre to asilicon block, the method comprising the steps of: forming a channelextending into the silicon block from a surface of the block, thechannel being sized so as to accommodate an end of the optical fibre;forming an aperture in the surface of the silicon block adjacent theopening of the channel in the surface of the silicon block; inserting anoptical fibre into the channel; and applying an adhesive to the opticalfibre and to the surface of the silicon block adjacent the optical fibreand including into the adjacent aperture.
 41. The method as claimed inclaim 40, wherein the aperture is an annular groove encircling theopening to the channel.
 42. The method as claimed in claim 40, whereinthe width of the aperture at the surface of the silicon block is lessthan the width of the aperture below the surface of the silicon block.